Ultraviolet light is that portion of the electromagnetic spectrum adjacent to the short wavelengths, or violet end, of the visible light range. The ultraviolet spectrum can be divided into two regions: the near ultraviolet (near the visible spectrum), with wavelengths 200 to 380 nm; and the far ultraviolet, with wavelengths 10 to 200 nm. Approximately ten percent of the energy from the sun is in the ultraviolet range.
The near ultraviolet spectrum can also be divided into three subregions: ultraviolet A which has wavelengths that are generally in the range of 320 to 380 nm; ultraviolet B which has wavelengths that are generally in the range of 280 to 320 nm; and ultraviolet C which has wavelengths that are generally in the range of 200 to 280 nm.
The solar ultraviolet intensity at the earth's surface depends upon a variety of factors including: the thickness of the ozone layer in the upper atmosphere, ozone absorbing ultraviolet radiation strongly between 200 and 300 nm; the latitude; the elevation above sea level; the atmospheric turbidity; the time of day; the time of year; and local conditions such as clouds, fog, dust, and smoke.
Human exposure to ultraviolet radiation can produce effects ranging from suntan, to sunburn, to skin cancer. While there are protective devices, including clothing, oil, ointments, and lotions, which either absorb or reflect ultraviolet radiation or reduce its penetration, there are no portable, inexpensive, disposable, readily available, simple, devices for quantitatively detecting the extent of exposure to ultraviolet radiation. Accordingly, it is desirable to have such a device which the consumer can easily use to ascertain the total amount of ultraviolet exposure.
Exposure to ultraviolet radiation can be measured either directly using electronic devices, such, as semiconductors, or indirectly using a spectrophotometer to quantitate the appearance or disappearance of a chemical species reactive to ultraviolet radiation.
The photochemically mediated detection or measurement of exposure to radiation, especially in the visible and ultraviolet range, is known as actinometry. A classical liquid phase actinometer is described in Hatchard et al., Proc. Royal. Soc., A235:518 (1956), based on the photoreduction of ferrioxalate to form free ferrous ion which reacts with 1,10-phenanthroline causing a green to red color change. Frankenburger et al., U.S. Pat. No. 1,845,835, discloses an ultraviolet sensitive solution, containing leucocyanides, carbinols, and sulfurous compounds, which undergoes a direct, one-step, reversible color change upon exposure to ultraviolet light. These systems will not specifically measure ultraviolet radiation as they are also sensitive to longer wavelengths.
Pavlickova et al., Col. Czechoslovak Chem. Comm., 51:368 (1986), describes a liquid phase actinometer, based on the photohydrolysis reaction of 3,4-dimethoxynitrobenzene, for the determination of ultraviolet light intensity. This liquid phase system does not have output in the visible range and is intended for use with a spectrophotometric device. Thus, utility in "field" applications is severely limited.
Reversible reactions have been suggested for use in actinometry. In the solution systems of Burg, U.S. Pat. No. 3,561,969, for example, a light sensitive solution undergoes a direct, one-step, reversible color change with short exposures to white light. Actinometric complications and resultant inaccuracies are introduced by reversibility in such systems.
Solid state actinometers which rely on transformations of compounds showing no absorption in the visible range frequently have the disadvantage of requiring use of a spectrophotometer. Examples of such systems are found in Bunce et al., J. Photochem., 23:219-231 (1983), and Bunce et al., J. Photochem., 34:105-115 (1986), which disclose the use of azoxybenzene in blocks of polymethylmethacrylate. Some solid state systems have the further disadvantage of requiring specially designed vacuum cells. For example, Cowell et al., J. Am. Chem. Soc., 90:1106 (1968), discloses nitrobenzaldehyde in a film of polymethylmethacrylate as part of a system also requiring a spectrophotometer.
Disadvantages associated with some actinometric systems include their variable sensitivity and inability to discriminate among various wavelengths of radiation--some systems being responsive to broad ranges of radiation and others limited to narrow ranges. Several patents (e.g., Smith, U.S. Pat. No. 3,929,488; Harper et al., U.S. Pat. No. 4,296,194; and Shelnut et al., U.S. Pat. No. 4,348,471) disclose a process that is used in lithographic print plate manufacture and is responsive to a mixture of both ultraviolet and visible light. Not only is the process not specific for ultraviolet radiation, it does not have a dynamic range of responsiveness, is extremely reactive to low levels of radiation, and utilizes unstable diazonium compounds which are dangerous if they come in contact with the skin. In contrast, Schmidt et al., J. Photochem., 25:489-499 (1984), describes two systems specific for two distinct regions of ultraviolet light. One system, the photoxidation of meso-diphenyl-helinathrene, is recommended for the 475-610 nm range and a second system, including the photoreversible photocycle reversion of the endoperoxide of heterocoerdianthrone, is suited for re-usable actinometry in the 248-334 nm range. Again, systems such as these that require a photometer are disadvantageous because their "output" cannot be directly visualized. Further, the compounds employed are both expensive and potentially carcinogenic.
Another system responsive to both visible and ultraviolet radiation utilizes photoactivators or photosensitizers which enhance the density of image formation upon exposure of leuco dyes to short periods of ultraviolet light without affecting the leuco dyes' reactivity to visible light. See, e.g., Sprague et al., U.S. Pat. No. 3,121,632. Yet another system responsive to both visible and ultraviolet radiation is disclosed in Wainer, U.S. Pat. No. 3,112,200. Upon a several seconds exposure to light (250-400 nm), the halogen-containing compound of the dry photographic film is converted to a free radical, which leads to the production of significant amounts of acid and water, thereby resulting in a visible color change in the acid-base type indicator dispersed throughout the film. Still another system responsive to both visible and ultraviolet radiation is disclosed in Zweig, U.S. Pat. No. 3,903,423. Zweig discloses two systems; the first uses oxzolidine-diones which darken only in response to radiation shorter than 320 nm; the second system uses photochromic cyclohexadiene compounds, such as xanthenones, which absorb not only from 320 nm and shorter, but also absorb at wavelengths longer than 320 nm, thereby requiring a filter to protect the system from longer wavelength radiation, such as visible light.
Some measurement systems which do not require a photometer to determine the amount of radiation exposure are simultaneously sensitive to both far ultraviolet radiation and to ionizing radiation and insensitive to near ultraviolet radiation and therefore cannot be used for the selective measurement of exposure to near ultraviolet radiation. For example, McLaughlin, Intl. J. of Applied Radiation and Isotopes, 17:85-96 (1966), discloses pre-activated colorless cyanides of triphenylmethane dyes, which can be made into films which, upon irradiation with far ultraviolet or ionizing radiation change from a colorless to colored state. See also, McLaughlin et al., U.S. Pat. No. 4,006,023. Similarly, Cerami et al., U.S. Pat. No. 4,466,941 discloses a composition comprising a complex of leucocyanide and serum albumin which, upon exposure to x-rays, gamma rays, and/or other short wave length radiation including, ultraviolet radiation, results in the appearance of color.
Systems for detecting ionizing radiation wherein exposure to ionizing radiation causes a halogen-containing compound to form a halo-acid which in turn causes an acid-sensitive dye to change color are disclosed in numerous patents. See, e.g., Vale et al., U.S. Pat. No. 3,290,499; Huebner et al., U.S. Pat. No. 3,691,380; Matsumoto et al., U.S. Pat. No. 3,743,846; Hori et al., U.S. Pat. No. 3,899,677; and Lemahieu et al., U.S. Pat. No. 4,008,085. In general, these systems are either heat sensitive or visible light sensitive. To the extent that they display ultraviolet light sensitivity, none are noted to be specific for ultraviolet light detection. None of these systems is suitable for use in applications requiring low toxicity.
Reversible photochromic materials useful in the preparation of photochromic plastic films, sheets, and opthalmic lenses and rapidly responsive to exposure to light are disclosed in Uhlmann et al., U.S. Pat. No. 4,012,232 and Wagner et al., U.S. Pat. No. 3,666,352. Generally, these materials change their transmission or reflectance upon being subjected to ultraviolet or visible irradiation and subsequently revert to their original state upon exposure to a different wavelength of radiation, or removal of the initial light source. Photochromic polymers capable of undergoing reversible changes between two chemical species induced by light absorption can be used for reversible optical information storage and have been suggested for actinometric uses in badges to detect unsafe levels of ultraviolet exposure. Wilson, Phys. Technol., 15:233 (1984). However, such a suggestion does not take into account the reversibility of the reaction nor the responsiveness of the reaction to visible light and resultant erroneous readings obtained when attempting to ascertain the extent of ultraviolet exposure.
Chem. Eng. & News, 64:77 (September 1986), reports that a system for indication of the passage of time and exposure to elevated temperatures is being developed as a spoilage indicator for perishable products. The indicators consist of filter paper dipped in a solution containing a leuco base dye and orthonitrobenzaldehyde. Commencing with a discrete photoactivation step, an oxidation process causes the indicator to begin to undergo a progressive color change in the acidified environment which is both time and temperature dependent. The rate at which the color change appears can be adjusted in accordance with the shelf-life of any given product so that a color change appears more quickly for some goods having a short shelf-life. Visible light above 400 nm does not activate the system. After initial photoactivation, the dyestuff color change reaction of this system is responsive to the passage of time and increased temperature; however, the system cannot measure the extent of subsequent exposure to ultraviolet radiation.
In sum, numerous actinometric devices and systems have been proposed in the prior art. None has been totally responsive to the need in the art for systems allowing ready visualization of cumulative exposure to ultraviolet light which are easily constituted from relatively inexpensive and non-toxic components and which display specificity for ultraviolet light and relative insensitivity to heat and the passage of time.